Effects of simulated microgravity on the microbial physiology of Ralstonia pickettii isolates from the International Space Station

Abstract

Bacterial growth in the potable water system aboard the International Space Station is currently controlled using iodine. While iodine has been shown to be an effective biocide, it can accumulate in the thyroid of some individuals, impart a bad taste to drinking water, and is incompatible with the biocidal silver system used in the Russian segment. In comparison, silver is effective as an antimicrobial at very low concentrations, poses health risks only at very high intake levels, and is readily excreted from the body. In addition to controlling bacteria‐related risks to crew and spacecraft health, biocide is also necessary to prevent interplanetary transport of microorganisms. In this study, we investigated the effects of various stress assays on the bacterial isolate Ralstonia pickettii in a simulated microgravity environment. Isolates were subjected to acid, thermal, and silver biocide stress assays at three different concentrations. A rotating wall vessel culture apparatus was used to provide a low‐shear culture environment, simulating certain aspects of spaceflight. This environment is known as low‐shear modeled microgravity (LSMMG). Isolates used in this study were originally identified by 16S ribosomal gene sequencing of return water samples from long‐duration expeditions 29 and 35 to the International Space Station. Isolates were grown in sterile water to simulate the potable water environment aboard the ISS. The thermal stress and silver biocide assays showed significant effectiveness in bacterial control, while the acid stress assay showed minimal effectiveness. All three concentrations of silver biocide produced a zero percent survival rate in both control and LSMMG environments. An increased growth rate in isolates grown in the LSMMG environment was shown. No differences between the control and LSMMG environments were evident in the response of R.pickettii to stress assays. A ten percent difference in survival rate was seen in the acid stress assay results, with LSMMG culture having a lower survival rate. These results suggest silver as a promising candidate to replace iodine as we move towards long duration spaceflight.